Part of the reason is that the non-modular is slightly better.The 650CX is semi-modular (for some reason the non-modular is $11 more). I am running this system: https://pcpartpicker.com/list/9kwn6R, and I will probably upgrade next year to a Ryzen, although that should actually use less power than this. My i7 920 is overclocked with a be Quiet Pure Rock to 4Ghz, but my current PSU is holding up, so I think it shouldn't be a problem with a new PSU around 550-650W. And my RX570 isn't exactly a power-hungry GPU.
The RX 570 isn't a hungry beast like, say, the old 280X, but it's quite power-hungry for its performance level. Some of the overclocked RX 570s at gaming can get up into the 160W range (and more if you're stress testing). That puts it right up near the GTX 1070 and 30-40W more than the 1660 Ti, which is a significantly more powerful GPU.
I get it, spending money on a power supply isn't sexy. There are more fun things to spend money on. Since it appears you're going to ask about every power supply cheaper than the least expensive PSU that everyone is recommending, all I can do is wish you good fortune and go help those who are actually seeking good advice.